Abstract:

Methods for ultrashort pulse laser processing of optically transparent
materials. A method for scribing transparent materials uses ultrashort
laser pulses to create multiple scribe features with a single pass of the
laser beam across the material, with at least one of the scribe features
being formed below the surface of the material. This enables clean
breaking of transparent materials at a higher speed than conventional
techniques. Slightly modifying the ultrashort pulse laser processing
conditions produces sub-surface marks. When properly arranged, these
marks are clearly visible with side-illumination and not clearly visible
without side-illumination. In addition, a method for welding transparent
materials uses ultrashort laser pulses to create a bond through localized
heating. The ultrashort pulse duration causes nonlinear absorption of the
laser radiation, and the high repetition rate of the laser causes
pulse-to-pulse accumulation of heat within the materials. The laser is
focused near the interface of the materials, generating a high energy
fluence at the region to be welded. This minimizes damage to the rest of
the material and enables fine weld lines.

Claims:

1. A method of scribing a transparent material, comprising:using a single
scan of a focused beam of ultrashort laser pulses to simultaneously
create a surface groove in said material and at least one modified region
within the bulk of said material.

2. A method for scribing a transparent material, comprising:using a single
scan of a focused beam of ultrashort laser pulses to simultaneously
create a plurality of modification regions within the bulk of said
material.

3. A transparent material scribed at two or more points in a depth
direction thereof by a single scan of a focused beam of ultrashort laser
pulses.

Description:

CROSS-REFERENCE TO RELATED APPLICATION

[0001]This is a divisional of application Ser. No. 11/517,325 filed Sep.
8, 2006, which claims benefit of Provisional Application No. 60/714,863
filed Sep. 8, 2005. The entire disclosure of the prior application,
application Ser. No. 11/517,325 is considered part of the disclosure of
the accompanying divisional application and is hereby incorporated by
reference.

[0006]Cutting of optically transparent materials is often done with
mechanical methods. Perhaps the most common method for cutting thin, flat
materials is using a mechanical dicing saw. This is the standard method
in the microelectronics industry for dicing silicon wafers. However, this
method generates significant debris that must be managed in order to
avoid parts contamination, resulting in increased overall cost of the
process. In addition, the thinner wafers being used for advanced
microprocessor designs tend to shatter when cut with a dicing saw.

[0007]To address these problems, current state-of-the-art processes for
"scribe and cleave" material cutting use various types of lasers to
scribe a surface groove on the material prior to breaking the material
along this scribe. For example, a sub-picosecond laser pulses have been
used to cut silicon and other semiconductor materials (H. Sawada,
"Substrate cutting method," U.S. Pat. No. 6,770,544). Also, a focused
astigmatic laser beam has been used to make a single surface groove for
material cutting (J. P. Sercel, "System and method for cutting using a
variable astigmatic focal beam spot," U.S. patent application No.
20040228004). This patent claims that by optimizing the astigmatic
focusing geometry, increased processing speeds can be achieved.

[0008]To achieve a precise, high quality cleave, the groove must be of a
certain minimum depth, the value of which varies by application (for
example, 100-μm thick sapphire requires an approximately 15-μm deep
groove for acceptable cleaving). Because the depth of the groove
decreases as the scanning speed increases, the minimum depth requirement
limits the maximum scanning speed, and hence the overall throughput, of a
laser-scribing system. Alternative technology for material cutting uses
multiphoton absorption to form a single laser-modified line feature
within the bulk of a transparent target material (F. Fukuyo et al.,
"Laser processing method and laser processing apparatus," U.S. patent
application No. 20050173387). As in the case of a surface groove, there
is a particular minimum size of this sub-surface feature that is required
in order to yield precise, high quality cleaving of the material, which
equates to a limit on the processing speed for material cutting.

[0009]A noteworthy application of "scribe and cleave" material cutting is
wafer dicing for separation of individual electronic and/or
optoelectronic devices. For example, sapphire wafer dicing is used in
singulation of blue light emitting diodes. Wafer singulation can be
accomplished with backside laser ablation, minimizing contamination of
devices on the front side of the wafer (T. P. Glenn et al., "Method of
singulation using laser cutting," U.S. Pat. No. 6,399,463). Also, an
assist gas can be used to aid a laser beam that dices a substrate (K.
Imoto et al., "Method and apparatus for dicing a substrate," U.S. Pat.
No. 5,916,460). In addition, a wafer can be diced by first using a laser
to scribe a surface groove, and then using a mechanical saw blade to
complete the cutting (N. C. Peng et al., "Wafer dicing device and
method," U.S. Pat. No. 6,737,606). Such applications are generally
executed in large volume and hence processing speed is of particular
importance.

[0010]One process uses two different types of lasers, one of which scribes
the material, and the other of which breaks the material (J. J. Xuan et
al., "Combined laser-scribing and laser-breaking for shaping of brittle
substrates," U.S. Pat. No. 6,744,009). A similar process uses a first
laser beam to generate a surface scribe line, and a second laser beam to
crack a non-metallic material into separate pieces (D. Choo et al.,
"Method and apparatus for cutting a non-metallic substrate using a laser
beam," U.S. Pat. No. 6,653,210). Two different laser beams for scribing
and cracking have also been used to cut a glass plate (K. You, "Apparatus
for cutting glass plate," International Patent Application No. WO
2004083133). Finally, a single laser beam has been used to scribe and
crack a material by focusing the laser beam near the top surface of the
material and moving the focus down through the material to near the
bottom surface while providing relative lateral motion between the
focused laser beam and the target material (J. J. Xuan et al., "Method
for laser-scribing brittle substrates and apparatus therefor," U.S. Pat.
No. 6,787,732).

[0011]B. Material Joining

[0012]The joining of two or more optically transparent materials, such as
glasses and plastics, is useful for applications in various industries.
The construction of any type of device in which optical transparency
allows or supplements functionality, or otherwise results in additional
value (e.g. aesthetic), could benefit from such a joining process. One
example is hermetic sealing of components where visual inspection is
needed (e.g. telecommunications and biomedical industries).

[0013]In some applications, conventional joining processes (e.g.
adhesives, mechanical joining) are inadequate. For example, many
adhesives might prove non-biocompatible in the case of biomedical implant
devices. For other devices, the adhesion simply may not be strong enough
for the particular application (e.g. high-pressure seals). For such
demands, laser welding offers an ideal solution.

[0014]In microfluidic systems, the sealing of individual, closely spaced
paths relative to each other with a cap piece that covers the entire
device would be desirable. Strong, tightly sealing joints can be
difficult to make with other methods due to the small contact region
between the different microfluidic paths. Laser welding can precisely
position the bonded regions between these microfluidic paths and provide
a tight seal.

[0016](1) use of a CO2 laser, the wavelength (˜10 μm) of
which is linearly absorbed by many optically-transparent materials, or

[0017](2) introduction of an additional material at the interface of the
transparent materials, which is specially designed to absorb the laser
radiation, thereby causing heating, melting, and fusing of the materials.

[0018]Both of these methods are limited in their functionality and/or
costly in their implementation.

[0019]A pulsed CO2 slab laser has been used to weld Pyrex to Pyrex,
and to bond polyimide and polyurethane to titanium and stainless steel
(H. J. Herfurth et al., "Joining Challenges in the Packaging of BioMEMS,"
Proceedings of ICALEO 2004). Also, fused quartz and other refractory
materials have been welded with a 10.6 μm CO2 laser (M. S. Piltch
et al., "Laser welding of fused quartz," U.S. Pat. No. 6,576,863). The
use of such CO2 lasers does not allow welding by focusing the beam
through a thick top layer material, since the laser radiation is absorbed
before it can reach the interface. An additional disadvantage is that the
large wavelength does not allow focusing the beam to a small spot,
thereby limiting its usefulness for creating small weld features on
micron scales.

[0020]Alternatively an absorbing layer that is transparent to the human
eye can be placed between two materials to be welded, such as polyamide
and acrylic (V. A. Kagan et al., "Advantages of Clearweld Technology for
Polyamides," Proceedings ICALEO 2002). A diode laser with line focusing
is then used for welding (T. Klotzbuecher et al., "Microclear--A Novel
Method for Diode Laser Welding of Transparent Micro Structured Polymer
Chips," Proceedings of ICALEO 2004). The dye layer is specially designed
to absorb the laser's wavelength (R. A. Sallavanti et al., "Visibly
transparent dyes for through-transmission laser welding," U.S. Pat. No.
6,656,315).

[0021]One welding process for bonding glass to glass or metal employs a
laser beam to melt a glass solder between the surfaces to be welded (M.
Klockhaus et al., "Method for welding the surfaces of materials," U.S.
Pat. No. 6,501,044). Also, two fibers can be welded together by using an
intermediary layer that is linearly absorbent to the laser wavelength (M.
K. Goldstein, "Photon welding optical fiber with ultra violet (UV) and
visible source," U.S. Pat. No. 6,663,297). Similarly, a fiber with a
plastic jacket can be laser-welded to a plastic ferrule by inserting an
absorbing intermediary layer (K. M. Pregitzer, "Method of attaching a
fiber optic connector," U.S. Pat. No. 6,804,439).

[0022]The use of an additional layer of an absorbing material has
significant drawbacks. The most obvious is the cost of purchasing or
fabricating a material that is appropriate for the process. A potentially
more costly issue is the increase in processing time associated with
incorporating this additional material into the manufacturing process.
Such costs would be expected to rise dramatically as the size of the
desired weld region becomes increasingly small, as would be the case with
biomedical implant devices. Another disadvantage of using an
intermediate, light-absorbing layer is that this layer may introduce
contaminants into the area to be sealed. In the case of a microfluidic
system, the light-absorbing layer would be in direct contact with the
fluid flowing through the channel.

[0023]One method for welding a transparent material to an absorbing
material is called through-transmission welding. In this method a laser
beam is focused through a transparent material and onto an absorbing
material, resulting in welding of the two materials (W. P. Barnes, "Low
expansion laser welding arrangement," U.S. Pat. No. 4,424,435). This
method has been used to weld plastics by directing polychromatic
radiation through a top transparent layer and focusing the radiation onto
a bottom absorbing layer (R. A. Grimm, "Plastic joining method," U.S.
Pat. No. 5,840,147; R. A. Grimm, "Joining method," U.S. Pat. No.
5,843,265). In another example of this method, a black molded material
that is transparent to the laser wavelength is welded to an adjacent
material or via an added welding assist material that absorbs the laser
wavelength (F. Reil, "Thermoplastic molding composition and its use for
laser welding," U.S. Pat. No. 6,759,458). Similarly, another method uses
at least two diode lasers in conjunction with a projection mask to weld
two materials, at least one of which is absorbent of the laser wavelength
(J. Chen et al., "Method and a device for heating at least two elements
by means of laser beams of high energy density," U.S. Pat. No.
6,417,481).

[0024]Another laser welding method performs successive scans of a laser
beam over the interface between two materials to incrementally heat the
materials until melting and welding occurs (J. Korte, "Method and
apparatus for welding," U.S. Pat. No. 6,444,946). In this patent one
material is transparent, while the other material is absorbent to the
laser wavelength. Finally, one method uses ultraviolet, laser, X-ray, and
synchrotron radiation to melt two pieces of material, and then brings
them into contact in order to form a weld (A. Neyer et al., "Method for
linking two plastic work pieces without using foreign matter," U.S. Pat.
No. 6,838,156).

[0025]Laser welding is disclosed for hermetic sealing of organic light
emitting diodes where there is at least one layer of organic material
between two substrates ("Method of fabrication of hermitically sealed
glass package", U.S. Patent Application Publication 20050199599).

[0026]Tamaki et al. discuss the use of 130-fs laser pulses at 1 kHz to
bond transparent material in "Welding of Transparent Materials Using
Femtosecond Laser Pulses", Japanese Journal of Applied Physics, Vol. 44,
No. 22, 2005. However, the material interaction of low repetition rate
ultrashort pulses (kHz) is known to be distinctly different compared to
high repetition rate ultrashort pulses (MHz) due to electron-phonon
coupling time constants and accumulation effects.

[0027]C. Sub-Surface Marking

[0028]The patterning of sub-surface marks in glass has been adapted by
artists to create 2-D portraits and 3-D sculptural works. These marks are
designed to be strongly visible under a wide range of conditions without
requiring external illumination.

[0029]Tightly focusing energy below the surface of optically transparent
materials can produce visible, radially propagating micro-cracks.
Long-pulse lasers are commonly used to create these marks. Several
patents discuss variation of the size and density of these radial cracks
to control the visibility of the subsequent pattern (U.S. Pat. Nos.
6,333,486, 6,734,389, 6,509,548, 7,060,933).

[0030]The visibility of the mark can be controlled by the crack density
around the central laser spot, rather than just the size of the mark
(U.S. Pat. No. 6,417,485, "Method and laser system controlling breakdown
process development and space structure of laser radiation for production
of high quality laser-induced damage images").

[0031]U.S. Pat. No. 6,426,480 ("Method and laser system for production of
high quality single-layer laser-induced damage portraits inside
transparent material") uses a single layer of smooth marks where
brightness is controlled by the spot density.

[0032]Increasing the pulse duration of the writing laser light will
increase the brightness of the mark (U.S. Pat. No. 6,720,521, "A method
for generating an area of laser-induced damage inside a transparent
material by controlling a special structure of a laser irradiation").

SUMMARY OF THE INVENTION

[0033]Through nonlinear absorption, ultrashort laser pulses can deposit
energy into an extremely well-defined region within the bulk of a
transparent material. Matching the laser properties and processing
conditions can produce a range of features, changes in the index of
refraction that enable optical waveguiding, melting and subsequent
bonding at an internal interface, or the formation of an optical defect
that scatters light.

[0034]The high repetition rate of the laser and significant pulse-to-pulse
overlap results in an additional interaction between the material
modification created by the previous laser exposure and the subsequent
pulses in the same region. The light diffracts around the pre-existing
modification and, through constructive interference, creates another spot
in the "shadow" of the previous modification, commonly known as the "spot
of Arago" or the "Poisson spot". The size and intensity of the spot
increases with distance, with the intensity asymptotically approaching
the input laser intensity.

[0035]One object of this invention is to enable clean breaking of
transparent materials at a higher speed compared to the conventional
technique. This is achieved by using ultrashort laser pulses to create
both a surface groove on the material and one or more laser-modified
regions within the bulk of the material (or, alternatively, multiple
sub-surface laser-modified features only), with only a single pass of the
beam across the material. Because multiple scribe features are created
simultaneously, located both on the surface and in the bulk of the
material, or in the bulk of the material only, the success of the
subsequent cleave is not necessarily dependent on surface groove depth.

[0036]During the cleaving process of a scribed material, the fracture
begins at the surface scribe feature and propagates down through the
material. If the surface groove is too shallow, the fracture will tend to
wander, resulting in low quality cleave facets and poor cleave process
precision. With the presence of additional scribe feature(s) within the
bulk of the material, however, the fracture is guided through the
material in the desired direction, resulting in higher cleaving precision
and facet quality than would be expected for the case of a shallow
surface scribe only.

[0037]If a sufficient portion of the bulk material is modified below the
surface, the fracture can begin from a sub-surface modified region and
propagate to adjacent modified regions through the bulk of the material,
without the need of a surface scribe line. Minimizing the size of, or
completely eliminating, the surface groove also reduces the debris
produced by the process that can contaminate the processing environment
or require more extensive post-process cleaning.

[0038]Another object of this invention is the generation of patterns of
sub-surface defects in transparent materials by focusing ultrashort laser
pulses below the surface. Slightly modifying the processing conditions
relative to scribing can produce optical defects below the surface that
scatter light. By controlling the characteristics and arrangement of
these defects, these patterns can be made to be clearly visible when
illuminated from the side, but difficult to see when there is no
illumination. This feature of sub-surface marking can be utilized for
indicator signs or lights for vehicles, warning signs or lights, or for
adding value (e.g., artistically) to a simple glass, etc. This technique
is distinct from known laser marking techniques which are designed in a
way such that the defects produced in the material are always clearly
visible.

[0039]In one embodiment of this invention, a pattern of optical defects
are created at different depths within the transparent material. Having
the marks at different depths prevents a "shadowing" effect where one
mark blocks the illuminating light from hitting subsequent marks. This
structure at the same time reduces the amount of scattering from ambient
illumination sources which are not directional, enhancing the contrast
between the on-off states. These defects can be discrete points or
extended lines.

[0040]The small size and smoother profile of these defects makes them less
visible when not illuminated. Also the substrate will be stronger and
less susceptible to crack propagation due to thermal or mechanical
stress, particularly with thin transparent materials. The small size also
allows for more discrete writing positions per unit thickness, increasing
the pattern resolution for a given thickness of transparent material.

[0041]There is a trade-off between the visibility of the mark when
illuminated and the invisibility of the marks without illumination. This
trade-off can be adjusted by controlling the illuminating light source
intensity, the size and smoothness of the marks and the spacing between
marks. The control parameters for the size of the marks include pulse
duration, fluence, and repetition rate and wavelength of the laser, and
depth and movement speed of the focus point within the material. It is
important to note that these parameters need to be adjusted for
transparent materials with different optical, thermal and mechanical
properties.

[0042]The desired pattern can be made up of a collection of discrete
pixels where each pixel is a collection of parallel lines. Utilizing
pixels enables creation of an over-all larger icon with fewer lines with
greater contrast in visibilities.

[0043]The sub-surface pattern can be illuminated by a properly focused
light source. Focusing is important to efficiently illuminate the pattern
and minimize stray light. This illuminating light can be delivered
directly from the light source if the distance between the light source
and the pattern is relatively short. If the distance is long, total
internal reflection between the top and bottom surfaces of the
transparent material can be used to guide the light.

[0044]Another alternative is to create optical waveguides in the
transparent material to deliver the light. An advantage of optical
waveguide delivery is that the path between the source and the pattern
need not be straight and/or short. For optical waveguide delivery, the
output end of the waveguide should be properly designed to illuminate the
desired pattern.

[0045]Two patterns in the same region can be distinguished separately and
controllably illuminated by two different light sources. The axis of the
illumination source for the respective pattern is perpendicular to the
marks which make up the pattern. In this way the maximum scattering (and
maximum visibility) from a particular illumination source can be selected
for only the designated pattern.

[0046]Another object of this invention is to enable bonding of two pieces
of clear material using a high repetition rate femtosecond pulse laser
with no supplemental bonding agent. Focusing a high repetition rate,
ultrafast laser beam at the contact area of two transparent material
pieces will create a bond by localized heating. The required repetition
rate for sufficient heat accumulation depends on many different process
variables, including pulse energy, beam focusing geometry, and the
physical properties of the particular material(s) to be welded. A
theoretical analysis of conditions affecting the femtosecond laser spot
bonding process emphasizes the determination of optimal focusing
conditions for the process (M. Sarkar et al., "Theoretical Analysis of
Focusing and Intensity Mechanisms for a Spot Bonding Process Using
Femtosecond Laser," IMECE2003-41906; 2003 ASME International Mechanical
Engineering Congress, November 2003, Washington, D.C., USA).

[0047]Due to nonlinear absorption of the laser radiation (caused by the
ultrashort pulse duration), and the pulse-to-pulse accumulation of heat
within the materials (caused by the high repetition rate), welding of
transparent materials can be achieved with a degree of simplicity,
flexibility, and effectiveness that is unparalleled in existing
alternative methods. The nonlinear absorption process allows for
concentration of the absorbed energy near the weld interface, which
minimizes damage, and therefore optical distortion, to the rest of the
material. Fine weld lines are possible when dense channels need to be
separated.

[0048]Further, an embodiment of the current invention enables the joining
by laser of two materials that are transparent to the wavelength of the
laser radiation by directing the focal region of a beam of
high-repetition rate, ultrashort pulses near to the interface of the
materials to be joined. The laser pulse repetition rate is between about
10 kHz and 50 MHz and the laser pulse duration is between about 50 fs and
500 ps. The laser pulse energy and beam focusing optics are chosen so as
to generate an energy fluence (energy per unit area) of more than about
0.01 J/cm2 at the region to be welded.

[0049]The optimal range of fluence for welding depends on the particular
materials to be welded. For transparent polymers (polycarbonate, PMMA
(polymethylmethacrylate), etc.), the required fluence is less than that
for glasses. This is due to the widely different physical properties of
the materials. For example, the melting temperature of PMMA is ˜150
degrees Celsius, while that for fused silica is ˜1585 degrees
Celsius. Therefore, significantly more laser fluence is required to melt
fused silica. Other important material properties include the heat
capacity and the thermal conductivity. The range of fluence for welding
of polymers is between about 0.01 and 10.0 J/cm2, while the
corresponding range for welding glasses is between about 1.0 and 100
J/cm2.

[0050]In general, welding requires that the two surfaces to be joined have
virtually no gap between them. An object of this invention is the
formation of a raised ridge at the interface between the two pieces to be
bonded, that bridges any gap between them. By focusing high repetition
rate fs pulses slightly below the surface, heating, melting, and pressure
can result in localized raising of the surface of the glass. These bumps
are 10's of nm to a few μm high. Where the energy deposited is not
sufficient to cause the raised ridge to bond to the mating piece, a
second pass of the laser at a slightly higher focus position will then
weld the ridge to the mating piece. If a single ridge is not tall enough
to bridge the gap, a second ridge on the upper mating surface can be
created.

[0051]In addition, welding of materials with varying degrees of linear
absorption can be achieved with this invention. While this invention uses
nonlinear absorption phenomena as the primary means to couple energy to
the material, it is appreciated that materials exhibiting some amount of
linear absorption of the irradiating laser pulses can also be welded
using methods presented herein. The significant aspect of linear
absorption as it relates to this invention is that for higher linear
absorption, the thickness of the material through which the beam can be
focused decreases. Furthermore, higher linear absorption decreases the
degree of localization of the weld feature.

[0052]The spatial distribution of the laser fluence can also affect the
weld quality. While typical laser processing involves focusing a Gaussian
laser beam to produce a smaller Gaussian laser beam, novel beam-shaping
methods may be used in order to improve upon the quality and/or
efficiency of a particular welding process. For example, transforming the
typical Gaussian fluence distribution into a spatially uniform fluence
distribution (known as a "flat-top" or "top-hat" intensity distribution)
may result in more uniform weld features.

[0053]The ultrashort nature of the pulses allows for coupling of the laser
energy into the transparent material via nonlinear absorption processes;
however, this alone does not allow for laser welding, as this process
does not generally result in heating of the material. It is the
additional aspect of a high pulse repetition rate, combined with a
particular range of other processing conditions, that allows for
accumulation of heat within the materials so that melting, and subsequent
cooling and joining, of the materials can be achieved.

[0054]Due to the absence of a supplemental bonding agent, processing time
and expense are reduced, contamination inside the device due to excess
bonding agent is eliminated, and fine dimensional tolerances can be
maintained. Bond points and lines can be very close to other features
without causing any interference. Also, very limited thermal distortion
of material adjacent to the weld area is possible due to the concentrated
fluence at the focal volume and the nonlinear absorption process.

BRIEF DESCRIPTION OF THE DRAWINGS

[0055]The invention will be more clearly understood from the following
description in conjunction with the accompanying drawings, wherein:

[0056]FIG. 1 is a diagram of a system used in a method for scribing
transparent materials according to one embodiment of the current
invention, where (a) shows the system configuration, and (b) shows a
detail view of the scribing and subsequent cleaving;

[0057]FIG. 2 is an illustration of the surface and bulk scribe features
that are generated by a focused Gaussian beam according to one embodiment
of the current invention;

[0058]FIG. 3 is a diagram of a system that uses an axicon lens to generate
multiple sub-surface scribe lines according to one embodiment of the
current invention;

[0059]FIG. 4 is an intensity contour plot of a focused Gaussian astigmatic
beam used in one embodiment of the current invention;

[0060]FIG. 5 is an illustration of a diffractive optical element (DOE)
used in one embodiment of the current invention;

[0061]FIG. 6 is a diagram of a system used in a method for welding
transparent materials according to one embodiment of the current
invention, where (a) shows the system schematic, and (b) is an enlarged
view showing the detail of beam focusing within the adjoining materials;

[0062]FIG. 7 is an illustration of the welding process where a raised
ridge is used to fill the gap between two pieces. (a) shows the gap, (b)
shows the ridge formed by focusing the laser beam slightly below the
surface of the lower piece, and (c) show the weld formed when the laser
focus is moved up to the interface between the raised ridge and the upper
piece to be bonded.

[0063]FIGS. 8-10 show illustrations of the sub-surface marking, wherein an
arrow mark has been used as an example of the markings possible according
to the invention.

[0064]FIG. 11 is an optical micrograph showing experimental results of one
embodiment of the current invention;

[0065]FIG. 12 is an image sequence showing a fused silica weld according
to one embodiment of the current invention. (a) shows the fused silica
before breaking apart the weld, (b) shows the bottom surface of the fused
silica after breaking apart the weld, and (c) shows the top surface of
the fused silica after breaking apart the weld.

[0066]FIGS. 13-15 are photos of a glass marked sample made according to
the present invention; and.

[0067]FIGS. 16a and 16b are photos of a prior art decorative article made
by laser marking using a long-pulse laser, and an individual mark
thereof.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0068]1. Ultrashort Pulse Laser Scribing

[0069]FIG. 1 illustrates one embodiment of the current invention, which is
a method for scribing transparent materials for subsequent cleaving. This
embodiment employs a laser system (1) producing a beam of ultrashort
laser pulses (2), an optical system (6) that generates a desired laser
beam intensity distribution, and a target material (7) to be scribed that
is transparent to the wavelength of the laser pulses. In addition, a
Z-axis stage (8) is used for beam focus position control (depth), and an
automated X-Y axis stage assembly (9) is generally required for moving
the work pieces (7) laterally relative to the focused laser beam.
Alternatively, the laser beam (2) could be moved relative to a stationary
target material with the use of scanning mirrors (3), (4), and (5).

[0070]The laser beam (2) is directed through the optical system (6), which
transforms the laser beam (2) to create a desired 3-dimensional intensity
distribution. Particular regions of the transformed laser beam have
sufficient intensity to cause ablation and/or modification of the target
material via nonlinear absorption processes. Material ablation generally
refers to the vaporization of material from intense laser radiation.
Material modification more broadly refers to a change in the physical
and/or chemical structure of the irradiated material, which can affect
the propagation of a crack through the material. Laser modification
generally requires lower optical intensities than laser ablation for a
particular material.

[0071]The transformed beam is directed toward the target transparent
material (7) to cause ablation/modification of the material (7) at
multiple determined locations, within and/or on the surface, of the
material (7). The ablated and/or modified regions are generally located
in the material (7) along the optical propagation axis and are separated
within the material (7) by a determined distance. The transformed beam
and the target material (7) are moved relative to each other, resulting
in the simultaneous generation of multiple laser-scribe features in the
material (7). The multiple scribe features allow for cleaving of the
material (7) with the application of suitable force(s) (See FIG. 1(b)).

[0072]FIG. 2 illustrates another embodiment of the current invention, in
which a laser beam (10) having a Gaussian spatial intensity distribution
is focused to create sufficient intensity for nonlinear absorption and
subsequent ablation or modification of the target material (11). The
region of tightest focus is positioned below the material's surface to a
chosen location within the bulk of the material (11). In addition, by
employing suitable focusing optics and laser pulse energy, a region of
intensity sufficient to cause material ablation is at the same time
generated on or near the surface of the material (11).

[0073]The important aspect is that the pulse energy and focusing geometry
are chosen such that there is sufficient intensity to simultaneously
cause ablation or modification not only within the bulk of the material
(where the focused beam waist is positioned), but also at another point
on the optical propagation axis prior to the beam waist (12) (either in
the bulk or on the surface of the material). When the laser pulses
encounter the target material (11), their high-intensity region (near the
center of the radial Gaussian intensity distribution) is absorbed
nonlinearly by the material and ablation or modification occurs. The
outer spatial regions of the laser beam (outer edges of the Gaussian
intensity distribution), however, are too low in intensity to be absorbed
by the material, and continue to propagate to the beam waist, located
further within the bulk of the material. At the beam waist location, the
beam diameter is small enough to once again generate sufficient intensity
for nonlinear absorption and subsequent laser modification to occur in
the bulk of the material.

[0074]A region directly below the surface ablation may also be modified by
diffraction and constructive interference of succeeding pulses after the
initial surface feature is created (spot of Arago). A relatively high
repetition rate laser source better enables this process at reasonable
translation speeds.

[0075]Under these focusing and pulse energy conditions, translation of the
material (11) relative to the laser beam (10) results in the simultaneous
generation of multiple laser-modified regions (i.e. a surface groove (13)
and one or more bulk modified regions (14), or two or more bulk modified
regions), which together allow for precise cleaving of the material.

[0076]FIG. 3 illustrates another embodiment of the current invention, in
which an axicon (cone-shaped) lens (20) is used to generate the multiple
internal scribe lines (21). When illuminated with a laser beam (22), the
axicon lens (20) creates what is known as a 0th-order Bessel beam. The
name arises from the fact that the mathematical description of the
optical intensity distribution in the plane normal to the axis of
propagation is defined by the 0th-order Bessel function, with the radial
position from the beam center being the independent variable. This beam
has the unique property of containing a high-intensity central beam spot
(23) that can propagate with the same small size for much larger
distances than for the case of a similarly-sized beam spot generated by
conventional focusing methods (i.e. much longer than the Rayleigh range
of a conventionally-focused beam). The central intensity field is
surrounded by a plurality of concentric rings of light (not shown), the
intensity of which decreases with increasing radius. Due to an inward
radial component of their propagation vector, these rings of light
continually reconstruct the small, central beam spot (25) as the Bessel
beam propagates. Therefore, a small, high intensity central beam spot
(23) can be generated that maintains its small diameter through the
entire thickness of a target material (24). Because of the extended range
of tight beam focusing, the Bessel beam is also commonly referred to as a
non-diffracting beam.

[0077]Because the outer rings reconstruct the intense center spot (23), if
the center spot (23) is intense enough to cause ablation of the material
at the surface (26), the rings (which have a larger diameter than the
ablated region) will converge to the center of the beam a short distance
later, causing reconstruction of the intense center beam spot, at which
point ablation or material modification can occur again. With proper
optical system design and sufficient pulse energy, this process of
ablation and subsequent beam reconstruction can repeat through the entire
bulk of the transparent material (24). Other optical components, such as
graded-index lenses and diffractive optical elements, can also be used to
generate Bessel beams.

[0078]In additional embodiments of this invention, alternative beam
intensity transforming techniques, well known to those skilled in the
art, are employed in the optical system of the invention to tailor the
beam intensity to generate multiple scribe lines in the target material.
One such method utilizes astigmatic beam focusing to create two distinct
regions of high optical intensity, separated by a determined distance.
FIG. 4 displays an intensity distribution plot of a focused astigmatic
Gaussian beam, in which the focal planes of the X and Y axes are
separated by a distance of 20 μm. Note the presence of two distinct
high intensity regions (distinguished by the constant-intensity contour
lines). When directed at the target material, these two regions can be
used to create multiple laser scribe features.

[0079]Another method for generating multiple scribe features in a
transparent material employs a diffractive optical element (DOE) that is
designed to generate multiple regions of high optical intensity at
different locations along the beam propagation axis. FIG. 5 illustrates
how such a DOE could function. These multiple intense regions, when
directed at the target material, generate multiple scribe features for
subsequent cleaving of the material.

[0080]For a variety of beam-focusing and/or intensity-mapping methods used
to generate multi-scribe ablation features, additional optical components
could be introduced to generate an elliptical component to the overall
beam shape. By orienting the elliptical beam such that the long axis is
parallel to the direction of beam scanning, higher scanning speeds can be
achieved. Higher scanning speeds can be achieved because the elliptical
beam shape allows for sufficient pulse-to-pulse overlap for the machining
of smooth and continuous scribe features (as opposed to dotted scribe
features resulting from spatially-separated pulses ablating the
material). While increased pulse overlap, and higher scanning speeds,
could also be achieved with a larger circular beam spot, this would at
the same time result in a wider scribe feature width, which is often
undesirable.

[0081]2. Ultrashort Pulse Laser Welding

[0082]Another embodiment of the current invention relates to a process for
laser-welding of transparent materials. As shown in FIG. 6, this
embodiment requires the use of a laser system (50) producing a beam of
ultrashort laser pulses (51) at a high repetition rate; a focusing
element (55) (e.g. lens, microscope objective) of sufficient focusing
power; and at least two materials (56) and (57) to be joined together, at
least one of which is transparent to the wavelength of the laser. In
addition, a beam focus positioning stage (58) is used to adjust the focus
position of the laser beam (51), and an automated motion stage assembly
(59) is generally required for moving the work pieces (56) and (57)
relative to the focused laser beam.

[0083]In this embodiment, the two materials ("top piece" (56) and "bottom
piece" (57)) to be laser-welded are placed in contact with each other to
create an interface with little or no gap between their surfaces; a
clamping force may or may not be applied to the two pieces. A lens (55)
is then positioned in the path of the laser beam to create a focal region
of high intensity laser radiation. The two transparent materials (56) and
(57) are positioned relative to the focused laser beam such that the beam
focal region spans the interface of the top piece (56) and the bottom
piece (57). With sufficient laser intensity, welding of the material
interface will occur. By moving the transparent materials (56) and (57)
relative to the beam focal region, while at the same time keeping the
interface of the materials (56) and (57) in close proximity to the beam
focal region, a determined length of laser welding can be achieved. In a
particularly unique application of this embodiment, the materials (56)
and (57) could be positioned such that the focused laser beam travels
through the top (transparent) piece (56) and forms the focal region near
to the interface of the top piece (56) and the bottom piece (57),
resulting in welding of the two materials.

[0084]Unlike other laser welding processes, the process of the invention
welds by utilizing primarily nonlinear absorption rather than linear
absorption. Because of this, there are unique properties in this welding
process. The nonlinear absorption is very intensity dependent so the
process can be limited to the focus of the laser beam. Thus the
absorption can be made to occur only deep within a transparent material
around the focus. Typically nonlinear absorption by ultrashort pulses
leads to plasma formation and very little (if any) heat deposition, thus
ablation with ultrafast lasers leads to a very small heat affected zone
(HAZ). However, by keeping the intensity low enough so ablation does not
occur but high enough for nonlinear absorption to occur, some heat is
deposited. If the repetition rate of the laser is increased sufficiently
then the heat can be accumulated sufficiently in the material to lead to
melting.

[0085]The laser system (50) emits an approximately collimated laser beam
(51) of pulses having a pulse duration in the range of about 200-500 fs
and a wavelength of about 1045 nm at a pulse repetition rate between 100
kHz and 5 MHz. The first beam steering mirror (52) directs the laser beam
to the power adjust assembly (53), which is used to adjust the pulse
energy that is used for the welding process; specific methods for
achieving such attenuation are well known to those skilled in the art.
The second beam-steering mirror (54) directs the beam onto the beam
focusing objective (55). The beam focusing objective (55) focuses the
laser pulses to achieve the appropriate fluence (energy/unit area) for
the process, which has a maximal value at approximately a distance (F)
from the beam focusing objective (55). The beam focus positioning stage
(58) translates the beam-focusing objective (55) such that this maximal
fluence region is located at the interface of the target materials (56)
and (57). The XY stage assembly (59) moves the target materials (56) and
(57) relative to the focused beam so as to provide for the ability to
generate a linear weld feature, or an array of circular weld features, at
the interface of the target materials (56) and (57).

[0086]FIG. 7 shows another embodiment of this invention where welding is
desired between two pieces separated by a small gap (60). First, the
laser beam (51) is focused below the surface of the bottom piece (57).
With the proper control of the pulse energy and focusing conditions, a
raised ridge (61) is formed as the sample is translated relative to the
beam focus (or as the beam is moved relative to the target). This raised
ridge (61) bridges the gap between the top and bottom targets. A second
pass of the laser with the beam focus raised to the height near the
interface between the top of the raised ridge (61) and the top piece (56)
then forms the weld (62).

[0087]3. Visible/Invisible Laser Marks

[0088]The same system shown in FIG. 1A can be used to make sub-surface
marks in transparent materials where the applied laser beam is focused
below the surface of the transparent material substrate

[0089]FIG. 8 shows an illustration of the top-view of an arrow pattern
(63) written in a transparent material (64) such as glass. A light source
(65) injects light into an optical waveguide (66) that delivers the light
to the arrow mark (63) to illuminate the pattern. The output numerical
aperture of the optical waveguide should be properly designed to
efficiently illuminate the desired source. Multiple optical waveguides
can be used to illuminate different regions of a pattern. Controlling the
timing of the different illuminating light sources can produce different
decorative and signaling effects. Alternatively, the pattern can be
illuminated directly from a properly focused light source, rather than
using an optical waveguide.

[0090]FIG. 9(a) shows an illustration of a close-up of the top-view of the
arrow mark (63) that is made up of parallel lines, all perpendicular to
the direction of the illumination light. These parallel lines are
generated by tightly focusing the laser light within the target substrate
to create regions of material modification. FIG. 9(b) shows an
illustration of the side-view of the arrow mark (63). The arrow mark is
composed of a group of marks at different depths. These marks scatter the
light delivered by the optical waveguide (66) towards the viewer (67).
The brightness can be controlled by the intensity of the illuminating
light, the size of the individual marks and the density of the marks.

[0091]FIG. 10 shows an illustration of the concept where the pattern is
composed of a "pixels" (68) and where each pixel is made up of a group of
parallel lines at different depths (69) formed by tightly focusing the
laser light to modify the substrate material.

Experimentally Demonstrated Results

[0092]1. Ultrashort Pulse Laser Scribing

[0093]As shown in FIG. 11, with a single pass of the laser beam, a pair of
scribe lines (a surface groove (70) and a sub-surface scribe feature
(71)) were simultaneously machined in a 100-μm thick sapphire wafer
using a 20× aspheric focusing objective (8-mm focal length). The
cleave facet exhibits good quality. The scanning speed was 40 mm/s (not
optimized).

[0094]For the case of a surface scribe line only, using the same laser
pulse energy and repetition rate, and under identical processing
conditions (ambient atmosphere environment, etc.), the fastest scribing
speed which resulted in good cleaving of the material was ˜20 mm/s.

[0095]2. Ultrashort Pulse Laser Welding

[0096]After a number of laser pulses are absorbed within a particular
region of the materials to be welded, heating, melting and mixing of the
materials occurs and, upon cooling, the separate materials are fused
together. The number of pulses required to weld the materials together
depends on other process variables (laser energy, pulse repetition rate,
focusing geometry, etc.), as well as the physical properties of the
materials. For example, materials with a combination of high thermal
conductivity and high melting temperature require higher pulse repetition
rates and lower translation speeds to achieve sufficient thermal
accumulation within the irradiated volume for welding to occur.

[0097]A. Polycarbonate Welding

[0098]Experiments with a high-repetition rate, femtosecond pulse laser
source operating at a pulse repetition rate of 200 kHz and having a
wavelength of 1045 nm have resulted in the laser-joining of two
optically-transparent materials. In particular, ˜2 μJ laser
pulses were focused with a 100 mm focal length lens through the top
surface of a 1/4''-thick piece of transparent polycarbonate, and onto its
bottom surface interface with the top surface of a similarly-sized piece
of transparent polycarbonate. The polycarbonate pieces were translated
linearly and in a plane perpendicular to the direction of laser
propagation, maintaining positioning of the beam focal region near-to the
interface of the materials. The two pieces were fused together at the
laser-irradiated interface and significant force was required to break
them free from one another.

[0099]B. Fused Silica Welding

[0100]A 200-μm thick fused silica plate was welded to a 1-mm thick
fused silica plate using a 40× aspheric lens and a laser repetition
rate of 5 MHz. The 1/e2 beam diameter of the laser was ˜3.6 mm
and the aspheric lens focal length was 4.5 mm, resulting in an operating
NA (numerical aperture) of 0.37. FIG. 12 shows a weld feature in fused
silica, with images taken both before and after breaking the two silica
plates apart. The first image (a) shows the intact weld feature
exhibiting regions of smooth melted glass, and the subsequent images (b)
and (c) show the two glass surfaces after the weld was fractured,
exhibiting facets of fractured glass.

[0101]Welding speeds ranged from 0.1 to 1.0 mm/s, though speeds greater
than 5 mm/s are possible, and the maximum speed could be increased with
an increased pulse repetition rate. The nominal fluence range for the
process is 5-15 J/cm2 and the nominal pulse duration range is
10-1000 fs. Within these fluence and pulse duration ranges, the nominal
pulse repetition rate range is 1-50 MHz. With rigorous process
optimization, these ranges may be extended to 1-100 J/cm2, 1 fs-500
ps, and 100 kHz-100 MHz for the fluence, pulse duration, and repetition
rate, respectively. The high repetition rate is required for sufficient
thermal accumulation for the onset of melting in the fused silica.

[0102]With the availability of higher energy pulses at similar repetition
rates, looser focusing is possible to generate a larger focal volume with
the required fluence. The size and shape of this welding focal volume can
be tailored based on the region to be welded.

[0103]3. Visible/Invisible Laser Marks

[0104]FIG. 13 shows a glass sample with the arrow mark illuminated by a
green light source from the side. Here, the arrow pattern is clearly
visible. The illustrations in FIGS. 8 and 9 show the details of the arrow
pattern, where lines at different depths, perpendicular to the
illuminating light source (green light in this case) were generated by
tightly focusing the laser light.

[0105]FIG. 14 shows the same glass sample with the illuminating light
source off. Clearly, the arrow pattern cannot be seen.

[0106]FIG. 15 shows a microscope photo of an individual pixel that is used
to define the arrow mark in FIG. 13. FIG. 16(a) shows a photo of a
decorative pattern inside glass and FIG. 16(b) shows a microscope image
of an individual mark.

[0107]The mark in FIG. 16(b) is approximately 200 μm in size and very
rough, composed of several distinct cracks radiating from the center. The
pixel in FIG. 15 is made up of a series of parallel lines, each line is
roughly 10 μm wide and 250 μm long. The line spacing is 50 μm.
The difference in size and smoothness difference between the features in
FIGS. 15 and 16(b) explains why the glass sculpture in FIG. 16(a) is
clearly visible in most lighting conditions while the arrow in FIGS. 13
and 14 requires side illumination to be visible. The size and smoothness
of the generated feature is controlled by the pulse energy, pulse
duration, wavelength of the laser and the translation speed of the beam
through the target. The optimal parameters depend on the particular
target material. The visibility of the pixel in FIG. 15 can be controlled
by controlling the width and length of each line in the pixel and the
line density within the pixel as well as the smoothness.

[0108]Thus, one method for generating visible patterns of laser-modified
features below the surface of the transparent material proceeds by first
forming a plurality of lines at different depths within the material
using a tightly focused ultrafast pulse laser, while controlling the
roughness of the lines by controlling parameters of said laser as
described. The lines are then illuminated using light propagating or
directed generally perpendicular to the lines. The patterns formed in
this way are clearly visible to the unaided eye when illuminated from the
perpendicular direction, although they are substantially invisible to the
unaided eye when not illuminated; i.e., under normal ambient light
conditions as in FIG. 14. The illumination is conducted by directing a
focused light source upon the lines or by directing the light to the
lines via an optical waveguide with an output numerical aperture selected
to efficiently illuminate the pattern.

[0109]Different ones of said lines, for example the lines of different
pixels, can be at defined angles relative to one another, and can be
illuminated separately or simultaneously by arranging multiple light
sources so that they each direct light generally perpendicular to a
subset of said lines.

[0110]Thus, the invention provides a transparent material having patterns
of sub-surface markings formed by a laser, e.g. an ultrafast pulse laser,
where the markings are formed of lines at different depths within the
material, with the lines being substantially visible to the unaided eye
only when illuminated with a light source directed generally
perpendicular to the lines.